The production of oxygen from air currently uses vacuum pressure swing adsorption (VPSA) or pressure swing adsorption (PSA) systems technology. These systems often have a capacity of less than 200 tons per day O2. Presently, there is renewed interest in extending the capacity of VPSA or PSA systems from such small scale (less than 200 tons per day O2) to large scale (about 350 tons per day O2 or higher) oxygen production from air.
In the application of VPSA or PSA processes, the energy input required to achieve the separation of O2 from the feed mixture (e.g., air) is provided as mechanical work through feed compressor(s) and vacuum pump(s). The cost of this work is a significant component of the total operating cost of the VPSA or PSA process. In addition, VPSA or PSA technology is currently economically competitive with cryogenic distillation only for small scale applications. In order for PSA or VPSA processes to become cost competitive with cryogenic distillation for large scale applications, improved cycles are required to operate the PSA or VPSA processes.
In applications where large scale oxygen production (e.g., 350 tons per day O2) is desirable, four bed VPSA processes are used. One such VPSA process is disclosed in U.S. Pat. No. 7,396,387 (Baksh et al.). U.S. Pat. No. 7,396,387 describes a four bed process which provides an efficient O2 VPSA cycle requiring lower power and hence delivering lower unit cost for O2 production. The four bed O2 VPSA system would need to employ 16 conventional double-ported automatic on/off valves to direct the flow of fluids at the feed and vacuum ends of the adsorbent beds. The capital cost of such a system with 16 conventional on/off valves offsets the benefits achieved by the four bed O2 VPSA cycle. Void volumes due to complex piping required by the 16 conventional on/off valves further reduce the benefits. The maintenance cost to service the 16 conventional on/off valves increases due to increased number of valves. Quick synchronous operations of the conventional on/off valves required by the new cycle imply additional costs.
Recently, U.S. Patent Application Publication No. 2008/0006151 (Baksh et al.) disclosed a polybed VPSA process and system to achieve enhanced O2 recovery. The VPSA process described therein uses three or more adsorber beds; providing a continuous feed using a single feed blower to one bed, wherein at any instant during the process, two beds are in an evacuation step and only one bed is in a feed mode; and purging the adsorber beds using two purge gases of different purity. According to the disclosure, about 10-20% improvement in O2 productivity is realized in the new VPSA process. However, the disclosed process uses 16 conventional on/off valves which increase the capital cost of the system.
U.S. Pat. No. 6,143,056 (Smolarek et al.) discloses a dual-chambered, four-ported rotary valve with a rotatable drum having peripheral openings. The valve is used to selectively connect vacuum, and/or air sources and a vent to adsorption chambers in a two bed VPSA system to replace separate valves and produce low cost enriched oxygen, via reduced power, increased reliability, and reduced complexity and cost. However, in accordance with the invention of U.S. Pat. No. 6,143,056, the rotary valve can only replace 8 conventional on/off valves when used in a four bed VPSA system. Thus, two such valves would be required to replace the 16 conventional on/off valves in a four bed VPSA system, requiring two driving mechanisms (i.e., one for each valve) and resulting in increased capital cost.
U.S. Pat. No. 6,253,778 (Smolarek et al.) discloses a rotary valve having first and second inlet/outlet flow passages, as well as one or more (preferably between 6 and 8) flow ports. The valve has a plurality of positions simultaneously interconnecting the first and second flow passage to selected flow ports. In a preferred embodiment, the valve has a housing and the flow ports are disposed on outer peripheral walls of the housing. The valve may further include an internal rotary plug that is rotatable about a longitudinal axis to move between the positions, and containing openings to connect the flow passages to the flow ports. The plug may be divided into two chambers in the valve which may or may not be moved independently of each other. The valve may further include a movable seal between the housing and the internal plug member which is retractable to permit freer movement of the plug. However, such a rotary valve will only replace 8 conventional on/off valves when used in a four bed VPSA system. Furthermore, two such valves would be required to replace the 16 conventional on/off valves in the four bed VPSA system, requiring two driving mechanisms (i.e., one for each valve) and resulting in increased capital cost.
U.S. Pat. No. 6,889,710 (Wagner) discloses a rotary sequencing valve comprising a rotor having a rotor face rotatable about an axis perpendicular to the rotor face, wherein the rotor face has a plurality of openings, one or more of which are disposed at a selected radial distance from the axis, and wherein the rotor includes at least one passage connecting at least one pair of the plurality of openings. The valve includes a flexible port plate having a first side and a second side, wherein the first side faces the rotor and engages the rotor such that the flexible port plate can be rotated coaxially by the rotor and can move axially with respect to the rotor, wherein the flexible port plate has a plurality of ports between the first and second sides, which ports are aligned with the openings in the rotor face. The valve also includes a stator having a stator face disposed coaxially with the rotor and the flexible port plate, wherein the second side of the flexible port plate is in sealable, slidable rotary contact with the stator face, wherein the stator face has a plurality of openings, some of which are disposed at the selected radial distance from the axis, and wherein the plurality of openings extend as passages through the stator. The valve may be used in pressure or temperature swing adsorption systems. However, the Wagner rotary valve is not suitable for large capacity O2 VPSA systems (i.e., approaching 200 tons per day O2) without becoming excessively expensive and prohibitively large in size and weight.
U.S. Pat. No. 6,936,091 (Beyreuther) discloses a valve assembly for use in a gas purification system having a plurality of vessels each having a first port opening and a second port opening. The gas purification system includes a first valve element having a first aperture to selectively connect a first port opening of a vessel to an outlet of the first valve element. The gas purification system also includes a second valve element having a second aperture to selectively connect a second port opening of a vessel to an input of the second valve element. Also provided are a motor adapted to rotate continuously and a converting mechanism that converts continuous movement of the motor into intermittent movement. The first and second valve elements are intermittently moved by the motor and the converting mechanism such that the intermittent movement changes the vessel connected to the second aperture and the vessel connected to the first aperture. However, in accordance with the teachings of U.S. Pat. No. 6,936,091, the rotary valve is not suitable for large capacity O2 VPSA systems (i.e., approaching 200 tons per day O2) without becoming excessively expensive and prohibitively large in size. In addition, the port size to accommodate larger capacities tends to increase the outer diameter and height of the valve, making it extremely large in size and weight.
U.S. Pat. No. 7,094,275 (Keefer et al.) discloses a rotary module for implementing a high frequency pressure swing adsorption process that comprises a stator and a rotor rotatably coupled to the stator. The stator includes a first stator valve surface, a second stator valve surface, a plurality of first function compartments opening into the first stator valve surface, and a plurality of second function compartments opening into the second stator valve surface. The rotor includes a first rotor valve surface in communication with the first stator valve surface, a second rotor valve surface in communication with the second stator valve surface, and a plurality of flow paths for receiving adsorbent material therein. Each flow path includes a pair of opposite ends, and a plurality of apertures provided in the rotor valve surfaces and in communication with the flow path ends and the function ports for cyclically exposing each flow path to a plurality of discrete pressure levels between the upper and lower pressures for maintaining uniform gas flow through the first and second function compartments. In accordance with the teachings of Keefer et al., the rotary valve is not suitable for large capacity O2 VPSA systems (e.g., 200 tons per day O2) without becoming excessively expensive and prohibitively large in size. In using the teachings of Keefer et al., for large VPSA O2 system, the moving parts including the adsorbent beds can become prohibitively heavy in systems of large capacity and can require substantial amount of power to rotate it; whereas, in the present invention, the drum valve can be used for 200 tons per day O2 VPSA systems and can be easily scaled up for larger capacity VPSA O2 systems (e.g., 350 tons per day of O2 or higher). In accordance with the teachings of the present invention, the weight and size of the drum valve are easily manageable and the moving parts of the drum valve are made of light materials (low density) for use in VPSA O2 systems using non-rotating or stationary adsorbent beds.
U.S. Pat. No. 7,276,107 (Baksh et al.) discloses a PSA system using an indexing rotary dual valve regulating a stepping mode of operation that controls a variable bed inlet feed flow rate, controllable pressure between feed lines in different beds of the PSA system and varied output flow rate of product gas such as high purity hydrogen gas. The rotary valve of U.S. Pat. No. 7,276,107 is not suitable for large capacity O2 VPSA systems (e.g., 200 tons per day O2) without becoming excessively expensive and prohibitively large in size. The port size to accommodate larger capacities tends to increase the outer diameter and height of the valve, making it extremely large in size and weight.
U.S. Patent Application Publication No. 2008/0000353 (Rarig et al.), discloses a pressure swing adsorption system comprising two or more vessels, each having a feed end, a product end, and adsorbent material adapted to adsorb one or more components from a multi-component feed gas mixture; piping adapted to (1) introduce the feed gas mixture into the feed ends, withdraw a product gas from the product ends, and withdraw a waste gas from the feed ends of the vessels, and (2) place the product ends of any pair of vessels in flow communication; a feed pipe adapted to supply the feed gas mixture to the system; a product pipe adapted to withdraw the product gas from the system; and a waste gas pipe adapted to withdraw the waste gas from the system. An indexed rotatable multi-port valve is adapted to place the product end of each vessel in sequential flow communication with the product end of each of the other vessels. However, as with the other prior art described herein, the rotary valve is not suitable for large capacity O2 VPSA systems (e.g., 200 tons per day O2) without becoming excessively expensive and prohibitively large in size. Also, the port size to accommodate larger capacities will increase the outer diameter and height of the valve, making it extremely large in size and weight.
Thus, a valve that can be used for a large VPSA system (e.g., 200 tons per day VPSA O2) and can be easily scaled up for larger capacity O2 VPSA systems (e.g., 350 tons per day of O2 or higher) is needed. In accordance with the design features of this invention, the weight and size of the drum valve are easily manageable versus prior art rotary valve designs. The present invention replaces the 16 conventional on/off valves in the Baksh et al., invention with one drum valve that performs the function of the 16 conventional on/off valves at a reduced capital cost. In addition, in the present invention, the drum valve also reduces the piping and void volume that was added due to use of 16 conventional on/off valves. Furthermore, the drum valve in the present invention results in reduction in maintenance costs and permits quick changes in cycle steps. Other features that reduce the cost of the present invention as compared to the prior art include but are not limited to the following: presence of a partitioned drum design which also adds to the structural integrity of the valve, presence of a modular seal design, presence of inexpensive sealing mechanisms, presence of inexpensive rotating and indexing mechanism.